The population and diversity of nitrogen-fixing bacteria in agricultural soils varies more according to what crop was previously farmed than with whether those soils are organically or conventionally farmed, according to a paper in the February 2011 issue of the journal Applied and Environmental Microbiology.

This study was conducted as part of the ongoing and long-standing Nafferton Factorial Systems Comparison study in Northumberland in northeast England, UK. The Nafferton study has conventional and organic plots side by side, enabling precise comparisons between the two methods.

In the study, the researchers analyzed soil samples from both sets of plots, once each in March, in June, after application of fertilizer (manure to organic plots, chemical fertilizer to conventional), and in September, following application of pest control measures.

“In general, you would expect organic fertilizers and pesticides to be less harmful than conventional ones,” says first author Caroline H. Orr of Northumbria University, Newcastle-upon-Tyne, UK. “However, we found that conventional fertility management led to a more active nitrogen-fixing community in June, directly after fertilizers were applied. This is possibly due to the positive effect of phosphorus, which is applied as part of conventional fertility management.”

However, in September, use of organic crop protection protocols led to more activity among the nitrogen fixing bacteria as compared to when conventional pesticides were used, says Orr, suggesting that “nitrogen-fixing bacteria are particularly sensitive to the toxic effects of chemical pesticides.”

It turns out the prior crop had a major influence on nitrogen-fixing activity, says Orr. Beans, legumes, commonly used in organic rotations have symbiotic relationships with bacteria that fix nitrogen for them; hence, they deplete less nitrogen from the soil, and the higher concentration of nitrate and ammonium suppresses the population of free-living nitrogen fixers. Conversely, soil growing barley (or other non-leguminous crops) would be relatively depleted of nitrogen, and so nitrogen-fixing bacteria would thrive, says Orr, remaining in higher numbers the following year.

The researchers assayed the population density of nitrogen fixing bacteria by measuring the concentration of nifH, the most conserved of the genes involved in nitrogen fixation. They sampled the density of all bacteria via 16S ribosomal RNA. The ribosomes are the machinery that reads ribonucleic acid to manufacture proteins.